The two major objectives of this proposal are to continue the investigation of the mechanisms underlying the maturation of HCO3- absorption in the juxtamedullary proximal convoluted tubule (JMPCT), and to initiate studies on the embryological origin and development of the acid-base transporting cell, the mitochondria rich (MR) cell, of the collecting duct. Absorption of HCO3- in the isolated perfused rabbit renal JMPCT has been found to surge to adult levels during the fifth and six weeks of postnatal life. This surge was not mediated by a rise in Na-K-ATPase activity, but may be due to an increase in number and activity of Na+-H+ exchangers, and be preceded by an increase in nephron GFR. The studies in this proposal will estimate the number of Na+-H+ exchangers in the luminal membrane by measuring both Na+-dependent H+ effluxes and cell pH during the same postnatal period. The studies in the collecting duct will focus on the function, development, regulation, and origin of the MR cell. In mature isolated perfused cortical and medullary collecting tubules, fluorescent assays will be developed to measure cell pH, probe for the presence of acidic vesicles, mitochondria, endocytosis, exocytosis and carbonic anhydrase, and assess surface binding of lectins. The results of these assays will permit one to distinguish between H+-secreting and HCO3-secreting types of MR cells. Additional studies of the function of the two cell types will assess the effects of ion substitutions on HCO3- transport, measured by microcalorimetry, and on cell pH, measured in individually identified cells with a pH-sensitive fluorescent dye. Because there may be interconversion between the two cell types, the frequency of each will be determined in cortical collecting tubules taken from rabbits subjected to chronic acid-base perturbations. Then the origin of MR cells will be traced back through fetal life to the mesonephric kidney with the same fluorescent methods to establish a fate map. Because the embryonic MR cell may be devoid of most functions that can be probed for with fluorescent dyes, monoclonal antibodies to the surface of the mature differentiated MR cell will be prepared. This antibody should help to identify the MR cell in renal and non-renal embryonic tissues even before differentiated functions appear.